1. Field of the Invention
The present invention relates to a process for treating a film comprising a liquid crystal polymer capable of forming an optically anisotropic melt phase, which comprises pressing it under specific conditions.
2. Description of the Related Art
Polymers capable of forming an optically anisotropic melt phase, i.e. what is known as thermotropic liquid crystal polymer, have regularity already in the melted state. Film formation by melt extrusion, such as T-die process and tubular film process, of the polymers therefore generally give films having large anisotropy in mechanical strength and elongation between the machine direction (hereinafter referred to as "MD") and a direction perpendicular thereto (hereinafter referred to as "TD"), whereby in extreme cases the films readily split in MD. Several processes have been proposed to reduce the anisotropy in the mechanical properties of films comprising thermotropic liquid crystal polymers. For example Japanese Patent Application Laid-open No. 45027/1983 proposes a process for orienting a film comprising a thermotropic liquid crystal polyester; which comprises passing the film through at least one pair of press rolls at a temperature in a range of from 100.degree. C. lower than the glass transition temperature of the film to a temperature lower than the flow temperature. According to the literature, the process can give films having good properties not only in MD but also in TD. Japanese Patent Application Laid-open No. 168327/1988 discloses a process for rolling a sheet which comprises passing a uniaxially oriented sheet comprising a liquid crystal polyester consisting of units from p-hydroxybenzoic acid, units from terephthalic acid and units from ethylene glycol between 2 pieces of rolls at a temperature at least the flow temperature and lower than the melting point of the liquid crystal polyester. The literature describes that a sheet thus rolled to a thickness of 95 to 50% of the original thickness can be provided with improved tensile strength and flexural modulus in TD without decreasing to a large extent the tensile strength and flexural modulus in MD.
As a result of a study made by the present inventors, it has been found that films obtained from liquid crystal polymers by melt extrusion generally have, in addition to large anisotropy in mechanical properties as described above, the drawback of small abrasion resistance of their surface, whereby the surface readily generate fluffs or naps by abrasion. These drawbacks have presented principal restrictions to development of end-uses. Besides, the present inventors attempted to apply the orienting process described in the above Japanese Patent Application Laid-open No. 45027/1983 and the rolling process described in the above Japanese Patent Application Laid-open No. 168327/1988 to films comprising liquid crystal polymers but could not obtain the intended rolled film having uniform thickness in a high yield, the processes being of poor reproducibility.
Accordingly, an object of the present invention is to provide a simple process for improving abrasion resistance and reducing the anisotropy in mechanical properties films comprising liquid crystal polymers.
Another object of the present invention is to provide a film having improved abrasion resistance and reduced anisotropy in mechanical properties.
Still another object of the present invention is to provide a use that makes the most of the features of the above film.